Ophthalmic lenses may be created using a variety of methods, one of which includes molding. In a double sided molding process, the lenses are manufactured between two molds without subsequent machining of the surfaces or edges. Such mold processes are described, for example in U.S. Pat. No. 6,113,817, which is expressly incorporated by reference as if fully set forth herein. As such, the geometry of the lens is determined by the geometry of the mold. Typical molding systems include cast molding, which involves using two mold halves, and spin-casting. These methods may also be combined with other machining techniques to create specific lens designs. Another process involves cycling lenses through a series of stations on a semi-continuous basis. The cyclic portion of lens production generally involves dispensing a liquid crosslinkable and/or polymerizable material into a female mold half, mating a male mold half to the female mold half, irradiating to crosslink and/or polymerize, separating the mold halves and removing the lens, packaging the lens, cleaning the mold halves and returning the mold halves to the dispensing position.
Once a mold is designed and fabricated it must be measured to ensure that it meets the proper specifications. Additionally, the mold material affects the end product, as the mold material may undergo non-uniform shrinkage as the mold is cured. Defects such cylinder and differential shrinkage are difficult to measure and characterize currently. The molded lenses must also be measured to ensure they are formed in the desired shape. The desired lens geometry may be spherical or non-spherical. The cured lens will not reflect the precise geometry of the mold due to volumetric shrinkage of the lens material. As the lens material cures, the arcuate surfaces of the lens result in a complex 3-dimensional change in the lens geometry from that of the mold.
The determination of the mold and lens geometries presents many challenges. A molded contact lens will distort under its own weight. The lens must then be supported by an optical tool to measure the lens geometry. The optical tool can distort the lens and result in an inaccurate measurement of the true lens geometry. Of particular difficulty is the measurement of the base curve of the lens. The base curve is the inner curved surface which contacts the eye. To compound the problems, the lens must also be kept hydrated during the measurement process to avoid shrinkage and distortion associated with the liquid content of the lens. Osmolarity, pH and temperature effects should also be considered or controlled when assessing the lens geometry.
Current techniques used to measure the lens and mold geometry include scanners such as vision, laser scan, interferometer, or touch probe. These techniques are difficult, slow, often inaccurate, and lack desired functionality. The lens or mold sample must be precisely positioned within the scanner or the accuracy of the measured geometry will be adversely affected. Most commercially available scanner technologies cannot capture a large area, such as the entire 14 mm diameter of a lens, and are only able to inspect a portion of the sample geometry at a time. The vision systems and lasers must have a direct line of sight with the surface being measured, which is not always possible on the arcuate surfaces of the contacts lenses, lens molds, and optical tooling. One example where direct line of sight is not possible is the base curve surface of a contact lens. The base curve surface is obscured from direct line of sight measurement by other portions of the surface. The touch probe scanning technique is a contact technique and involves correcting for induced changes in geometry as the compliant lens is deformed by the probe. Because of the direct line of sight or access requirement, the current techniques cannot inspect an object within an object, such as a lens clamped within a mold assembly.
Computed Tomography (CT) scanning is a well accepted method of medical imaging. The method uses a source of electromagnetic radiation, typically X rays, and a detector. An object to be scanned is positioned between the radiation source and the detector such that a portion of the electromagnetic radiation passes through the object before being received by the detector. The intensity of scattered and transmitted electromagnetic radiation is then measured at each pixel of the detector. The radiation intensity values at each pixel are then processed to form an image of the object being scanned. The source and detector are rotated through a specific path around the object being scanned and a number of X-ray images are collected. The intensity values for each image are then processed on a computer and utilizing the geometrical relationship between the source and detector, object surfaces are reconstructed to create a three dimensional geometric model of the scanned object. In an alternative approach, the sample is rotated relative to the source and detector while the X-ray images are taken. This approach is generally referred to as micro-Computed Tomography or microCT. In recent years, the resolutions of the resulting scans has increased to anywhere from 15 microns to 150 nanometers using high resolution, low cost imaging chips and the speed of the reconstruction of the data has greatly increased with faster computers.